1 Field of the Invention
The present invention relates to an electrode for a capacitor, a method for producing the same, and a capacitor. In particular, the present invention relates to a capacitor having good high-electric current properties and a large capacitance.
2 Prior Art
One example of capacitors having a large capacitance is an electric double layer capacitor. An electric double layer capacitor utilizes the electricity-storage function of the electric double layer formed at an interface between a polarizable electrode and an electrolyte. In general, an electric double layer capacitor comprises a pair of polarizable electrodes (a positive electrode and a negative electrode), a liquid electrolyte impregnated in the electrodes, a porous separator for separating the electrodes, which is impregnated with the liquid electrolyte and ion-permeable, and has an electrical insulation property to prevent the formation of a short circuit, and collectors connected with the respective electrodes.
Recently, it has been tried to use capacitors having a large capacitance as load leveling power sources of secondary batteries for electric vehicles by making use of the high-electric current charging and discharging characteristics of such capacitors. when the capacitors having a large capacitance are used as load leveling power sources, the voltage fluctuation of the secondary batteries is minimized when the vehicles are started or rapidly accelerated, or when the braking energy is recovered. Thus, the life of the secondary batteries is significantly prolonged.
When the capacitors having a large capacity are used as load leveling power sources, a voltage of 240 to 300 V, which is necessary to drive a motor, should be attained. To this end, it is necessary to connect 80 to 120 unit batteries in the case of capacitors using an organic liquid electrolyte, or 300 to 380 unit batteries in the case of capacitors using an aqueous liquid electrode, in series. Thus, the former capacitors using an organic liquid electrolyte are more advantageous than the latter ones using an aqueous liquid electrolyte, because of the smaller number of the unit batteries which are connected in series. However, in the case of organic liquid electrolyte type capacitors having a larger resistance than aqueous liquid electrolyte ones, a film having a high collection efficiency such as an aluminum foil should be used as a collector, and an opposed area of an electrode should be increased by forming a thin layer of a polarizable electrode material (polarizable electrode layer) on such a film, since a current of 50 to 200 A should flow through the capacitors.
When an electrode for such a capacitor is produced, a metal foil, a conductive polymer film or a carbon film is used as a collector, and a layer of a polarizable electrode material comprising activated carbon, a conducting aid and a binder is formed on the collector. To form the layer of the polarizable electrode material, following techniques are known:
a) Polytetrafluoroethylene (PTFE) as a binder, activated carbon powder and a conducting aid are kneaded to obtain a rubbery compound, and the compound is extruded around a mesh type collector such as an expanded aluminum sheet to form the layer of the polarizable electrode material. PA1 b) Polyvinylidene fluoride (PVdF) as a binder, activated carbon powder and a conducting aid are kneaded to obtain a coating composition for forming a layer of a polarizable electrode material, and applying such a composition on a collector such as an aluminum foil. PA1 c) A water-soluble binder (e.g. hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, carboxycellulose, etc.), an activated carbon fiber and a conductive aid are mixed to obtain a coating composition for forming a layer of a polarizable electrode material, and applying the composition on a collector such as an aluminum foil (see JP-A-4-14209) PA1 1) a thin coated film of a polarizable electrode material can be formed on a collector. PA1 2) the coated film is flexible so that the collector carrying the coated film can be wound in a small diameter. PA1 3) a coating composition comprising the binder system has a long pot life. PA1 dispersing activated carbon powder and a conducting aid in an aqueous medium containing polytetrafluoroethylene and hydroxyalkylcellulose dispersed therein,] PA1 applying such a dispersion on a collector, and PA1 drying the applied dispersion. PA1 Thirdly, the present invention provides a capacitor comprising PA1 a pair of electrodes each of which comprises a collector and a layer of a polarizable electrode material containing activated carbon powder, a conducting aid and a mixture of polytetrafluoroethylene and hydroxyalkylcellulose as a binder, PA1 a separator provided between a pair of the electrodes, PA1 an organic liquid electrolyte which is impregnated in the electrodes and the separator, and PA1 a container which contains the above elements.
An electrode having a layer of a polarizable electrode material, which is thinly and uniformly formed on a collector, is preferably used as an electrode for a capacitor having a large capacitance. When such a layer of a polarizable electrode material is formed, it is preferable to use PTFE having good heat resistance and electrochemical stability like in the case of the above a). It is also preferable to prepare a coating composition for the formation of a layer of a polarizable electrode material and then coating such a composition on a collector like in the case of the above b) or c), since the thin and uniform film of the polarizable electrode material can be effectively and easily formed. Thus, it may be possible to effectively form a layer of a polarizable electrode material having good properties, if a thin and uniform layer of a polarizable electrode material is formed by preparing a coating composition for the formation of a layer of a polarizable electrode material using PTFE as a binder, and applying such a coating composition on a collector.
However, PTFE has low thickening and adhesion properties and, therefore, no physical stability necessary for a coating composition is attained, when PTFE is used to prepare such a coating composition. Thus, it has been difficult to prepare a coating composition from a mixture containing PTFE. For this reason, the mixture is used in the form of a compound rather than a coating composition in the above a) . In this case, a mesh type collector such as an expanded aluminum sheet should be used, and thus the thickness of the layer of the polarizable electrode material increases. Therefore, an internal resistance of a capacitor tends to increase, and it may be difficult to attain a large electric current.
On the other hand, the above techniques b) and c) have no problem that arises when PTFE is used, since they use a binder comprising PVdF and a water-soluble binder, respectively, but they cannot use good properties of PTFE such as heat resistance. Furthermore, the above technique b) has some drawbacks such that a capacitor may have a low dielectric strength since N-methylpyrrolidone (NMP), which is used to dissolve PVdF, cannot be completely removed and remains in the liquid electrolyte, and that PVdF is easily decomposed during drying and thus the life of a coating and drying apparatus tends to be shortened, although the thin layer of the polarizable electrode material can be formed on the collector.